1. Field of the Invention
The invention relates to a method of fastening a circuit assembly to a light guide in a manufacturing process of a LCD-display, and a light guide for distributing indirect light in a LCD-display. The invention further relates to a LCD-display and a device.
2. Description of the Related Art
Liquid crystal displays (LCD-displays) are known in the field. A LCD-display comprises a number of pixel elements, usually arranged in a matrix formation, wherein each pixel element may be controlled individually to transmit and/or reflect light or not. By selectively controlling each pixel, together the pixels may form a (moving) image.
Each pixel may be divided in e.g. three sub-pixels, each sub-pixel comprising a different color filter (e.g. red, green, blue), thereby allowing the creation of color images.
The LCD-display may comprise image forming layers, such as a LC-layer comprising an array of liquid crystal elements, two polarizing layers (one on each side of the LC-layer), two electrode layers (one on each side of the LC-layer) arranged to address specific pixels by applying a voltage to the corresponding part of the LC-layer, and a color filter layer to provide different (sub)-pixels with different colors.
The skilled person will understand that other layers and elements may be provided to form a LCD-display. As will be understood, these image forming layers as described so far basically provide a shutter function, i.e. are arranged to transmit and/or reflect or block light, possibly with a certain color, for a specific (sub)-pixel.
To actually form an image, a light source is required. Different types of light sources may be used, such as (possibly used in combination): ambient LCD-displays, that form an image by selectively reflecting ambient light; direct back light LCD-displays, that form an image by selectively transmitting light emitted by a light source positioned behind the image forming layers (as seen from the point of perspective of a viewer); and indirect back light LCD-displays.
Indirect back light LCD-displays use one or more dedicated light source. The light source may be any type of suitable light source, such as a LED (Light Emitting Diode).
The light generated by this light source is distributed evenly over the surface of the image forming layers and travels through the image forming layers or is blocked by the image forming layers to form an image.
The indirect back light LCD displays use light from one or more light sources provided along the edge of the LCD-display, the light being distributed over the image forming layers via a light guide.
In case indirect back light is used, the light guide is positioned behind the image forming layers (seen from a viewer's point of perspective).
An example of a LCD-display using indirect light is shown in FIG. 1a, schematically showing a front view of an indirect back light LCD-display 1 as seen from a users point of perspective, comprising image forming layers 10 and a number of indirect light sources 20 positioned along the edge of the indirect back light LCD-display 1.
FIG. 1a further shows a circuit assembly 40, which may be a flexible printed circuit assembly (FPCA). The circuit assembly is shown in more detail in FIG. 1c. Such a circuit assembly 40 comprises the at least one light source 20 and is further provided with functionality to control the at least one light source 20 and the electrode layers to address specific pixels by applying a voltage to the corresponding part of the LC-layer. The circuit assembly 40 may further be arranged to receive an image signal based on which the control function can be performed.
FIG. 1b shows a cross-sectional view of an indirect back light LCD-display 1, as shown in FIG. 1a. In FIG. 1b a normal viewing direction of a user is indicated by arrow VD.
FIG. 1b schematically depicts a cross sectional view of an indirect back light LCD-display 1. The figure shows a light source 20, here used as an indirect back light source, emitting light into a light guide 30 which distributes the light to image forming layers 10. The light source 20 may be provided in a housing 21.
The light guide 30 comprises a first side 35 facing the image forming layers 10 and a second side 36, opposite the first side 35. The second side 36 may comprise a reflective layer 32 to prevent light loss.
The light guide 30 may be a layer made of plastic, such as poly-carbonate. The light guide 30 may be an optical transparent layer, plate or film (made of e.g. poly-carbonate), in which light is transported which enters the light guide 30 at an in-coupling side 34, which is facing a light emission window 22 of the light source 20.
The light guide 30 may comprise an out-coupling structure 31, provided on the first side 35, facing the image forming layers 10. The out-coupling structure 31 may be a structure formed by out-coupling features, such as prisms, dots or any other kind of surface corrugation which enables the out-coupling of light from the light guide 30. By varying the density of these out-coupling features, an even light distribution can be obtained.
FIG. 1d shows a light guide 30. As can be seen in FIG. 1d, the light guide 30 may be provided with a serrated edge, comprising recesses 38 and protrusions 37. The recesses 38 and protrusions 37 may be arranged to engage with the housings 21 of the light sources 20.
It will be understood by a skilled person that the image forming layers 10 may be used to generate an image. The electrode layers are used to apply a voltage over the liquid crystal layer, where a different voltage may be applied per pixel of the image to be formed. The voltage influences the orientation of the liquid crystal molecules.
Light traveling through the image forming layers will be polarized by a first polarizing layer. Next, the light passes through the liquid crystal layer where its direction of polarization may be altered by the liquid crystal layer depending on the orientation of the liquid crystal molecules (and thus the applied voltage). Next, the light meets a second polarizing layer. Depending on the polarization direction of the light when leaving the liquid crystal layer, the light will be able to (partially) pass the second polarizing layer. This allows controlling the light intensity for each pixel individually, thereby allowing forming an image.
The color filter layer may be arranged to provide a certain pixel with a certain color. The color filter may be omitted in case of a black-and-white liquid crystal display.
A step in the manufacturing process of a LCD-display is to fasten the circuit assembly 40, comprising the light sources 20, to the light guide 30.
The market drives the reduction of the “dead volume” and more in particular the thickness of LCD-displays. As a consequence the light guide 30 and light sources 20 are becoming thinner. Furthermore, the alignment and fixation between the circuit assembly 40 comprising the light sources 20 and the light guide 30 are becoming more critical.
A known technique to fasten the circuit assembly 40 comprising the light sources 20 to the light guide 30 is to use double sided adhesive tape. However, adhesive tape has a certain thickness which is becoming more and more disadvantageous for use in relatively thin LCD-displays. Also, the adhesive quality of adhesive tape may deteriorate under the influence of time and climate.
Another known technique to fasten the circuit assembly 40 comprising the light sources 20 to the light guide 30 is to use glue. However, applying glue in the manufacturing process is a cumbersome process step. Also, using glue in the manufacturing process may contaminate the manufacturing environment.